I have some imp001 projects where impOS 36's Fast Start functionality would be very useful, as I'm currently integrating a small LiPo charge+backup addition to the device to keep the imp powered while in deep sleep to facilitate a fast response when the device is 'powered on' via a lightswitch.

If I understand correctly, since imp.setrescuepin() is a factory firmware call it is only available to developers with SLAs and factory blessing? Is there any way for hobbyists to use this functionality?

Not right now; the issue is that this feature really lets you shoot yourself in the foot badly, which is why it's a bit hidden away.

You could use a simpler backup solution (eg a CR2 primary cell, couple of diodes - the drop will be negligible when in sleep from the battery and presumably the lightswitch power supply won't mind the power loss).